Jigglypuff's Final Smash. It wills it's body to get bigger and bigger and bigger. It can't move, and it can't attack-all it can do is get bigger while everybody else looks on. However, it is invulnerable, and it overpowers all attacks to gradually push it's opponents off the screen. It's a very simple technique, but how it's used determines on it's effectiveness.
There is a glitch that allows Jigglypuff to remain this size on certain stages, though Jigglypuff will be sent offscreen for a KO if it moves around too much.